It has been my expierience that the cannister round will open shortly after exiting the muzzle. The pattern is in a cone shape and expands as it goes downrange.As a result, some balls on the bottom of the cone strike the ground closer to the gun (about 80 yrds)and then bounce back into the air and continue on. This action continues as the balls lose velocity on out to about 350 yrds. I found spent cannister shot (1" lead balls) laying on the dirt from about 300 to 450 yds from the gun. The pattern opened to about 25 yrds across.
The targets have been both 6 Military "E" silhouette's placed shoulder to shoulder, and 4'x 8' plaster board distanced at 150 yards.Aiming directly at these targets and using authentic copies of cannister, there were oftentimes 3-4 of the silhouette's with one or more impact holes, and a surprisingly large areas of the plaster board that showed no damage from cannister shot.
I am therefore of the opinion that inspite of the presumed leathality of cannister, there is really a lot of "air" around a target and even at pointblank there would be a surprisingly number of unscathed infantry that would survive a cannister blast.
This opinion is based upon the effects of one gun firing a single round of cannister. If other gun(s) firing cannister were added to the situation, then the results would be substancally different.
Another interesting observation that I made is that when cannister is fired, the color of the gunsmoke changes. Instead of the usual cloud of white/gray smoke that occures when firing blackpowder, the smoke from cannister rounds was a yellow/orange/gray color. This is a result of the combustion of the wood sawdust that is packed inside the round. It wouldn't take long for a soldier (who survived) to associate the different color smoke comming from the cannon with the effects of cannister and to try and take action that might provide a better chance of survival.
As to the number of rounds per ammo load, that would be subjective to the desires of the Battery commander and the kinds that were available at resupply. Regulations call for specific numbers and kinds of ammo, but the reality of supply in the field might find a different mix of ammo in the chest.
Also, keep in mind that each gun & limber had a Cassion & limber,each with a mix of ammo so within the battery there would be a quick supply of cannister should the demand arise.